1. Field of the Invention
The present invention relates to a mount for an electric lamp having an elongated tubular portion. More particularly, the present invention relates to an electrically non-conductive mount for a miniature arc discharge lamp having an elongated tubular portion, said mount comprising a base and a cap assembled in mating engagement for exerting a force on at least one deformable elastomeric sleeve or ring shaped member. Still more particularly, the present invention relates to a lamp and mount assembly, said mount comprising a base and a cap assembled in mating engagement and said lamp comprising a miniature metal halide arc discharge lamp having an elongated tubular portion which is held in a bore in said mount by means of at least one elastomeric member therein which is compressed around said tubular lamp portion by cooperation between said base and said cap.
2. Background of the Disclosure
There is much interest in the automobile industry in using tungsten-halogen and arc lamps as the light sources for automotive headlamps. Tungsten-halogen lamps are presently used, but arc lamps have potentially longer life, higher light output and the size of arc lamps, such as metal halide arc discharge lamps, required for such lighting applications is relatively small, thereby enabling automotive manufacturers a greater leeway in innovative automotive design. Tungsten-halogen lamps presently employed for automotive lighting in standard replaceable or sealed beam headlamp units are generally welded to formed wires or posts which are then soldered or brazed to the lamp reflector through electrical feed-through members. Federal regulations are very stringent with regard to strength requirements for lamp sources for replaceable or composite lamps. Accordingly, such lamps are usually retained to a fixture by means of a strap member which is then welded to a metal member for the purpose of focusing and retaining the lamp in the base and in the reflector. U.S. Pat. No. 4,470,104 discloses a means for mounting a tungsten-halogen lamp wherein the lamp, due to temperature and other considerations is held in place by metal members. Still another means for mounting a tungsten halogen lamp in an automotive type lamp assembly is disclosed in U.S. Pat. No. 4,754,373 in which the lamp is held in place by metal members proximate to the lamp.
In contrast to tungsten-halogen lamps, arc discharge lamps, such as metal halide arc discharge lamps, require extremely high starting voltages, usually in the range of 10,000 to 20,000 volts. Because of these high voltages, it is necessary to electrically isolate the hot lead wire which exits the quartz or glass lamp envelope. Additionally, some of these lamp designs require very high starting frequencies in the order of 50 kHz in order to initiate the arc and at high frequencies metallic parts in the proximity of the hot lead wire tends to increase the capacitance of the system. The result of this increased capacitance is to decrease the level of voltage delivered to the lamp for the purpose of initiating the arc. Further, corona discharge sometimes occurs between the hot lead and metal parts proximate to the lamp or hot lead. Accordingly, it is desirable to limit the capacitance of the system by removing all but absolutely essential metallic elements from around the lamp. The use of metallic straps around the arc tube or in general proximity to the arc or high voltage lead would reduce the ability of the lamp to start and require higher voltages and more expensive electronics for starting a lamp in order to compensate for capacitance losses.
Still another phenomena which complicates the use of a scheme for supporting a metal arc discharge lamp relates to sodium loss from the arc chamber. Most arc tubes require compounds of sodium and one or more halogens to enhance their efficiency. Under certain conditions sodium ions can migrate through the quartz or high temperature glass arc chamber walls and the corresponding loss of sodium in the lamp results not only in hard starting or failure to start but darkening of the lamp envelope. Sodium migration out of the arc chamber also seems to be enhanced by the presence of metals near the arc chamber. This is a well known phenomenon in the lamp industry and larger metal halide lamps are designed to avoid or minimize the presence of metal near the arc chamber.